The dialogue that best helps develop both characters for readers is:
“We’re going to have so much fun,” Sadie said. “I bet we’ll get to meet the band. I bet we’ll even get to hang out with them!”
“We probably won’t even get to go,” Marco said. “I bet the whole show will get canceled and we’ll have a terrible day.”
This exchange effectively highlights Sadie's optimistic nature as she excitedly envisions all the fun they will have, while Marco's response illustrates his skeptical attitude and tendency to focus on the negative. Their contrasting perspectives on the same situation provide a clear and engaging development of their personalities.